No, that didn't help. It never goes full-screen. I waited some time anyway, but no :( I hear a sound and when the mouse pointer is over the window, it is like a bird or bat or so, flapping its wings. Everything as it should (I guess) except that a big part of it is out of sight.
I, too, guess it is a Godot problem.
I could report it as a bug to Godot, but I doubt they will accept a bug report like "Xau doesn't run".
(BTW it's on Linux.)